Need to prevent outlook from sending mail on behalf of another person within company

Dear All,

We already installed and configure Zimbra 8.0.0 as a Mail server,and use it with MS Outlook 2007 in our company.
We discovered a big issue, anyone can send mail on behalf of another person within company without any request or permission.
Please advice how we can disable on behalf feature on the Zimbra mail server? is there any way?